Images available through TerraLook provide snapshots of the Earth's surface for circa 1975, 1990, 2000, 2005, and the present. The 1975 dataset uses Landsat MultiSpectral Scanner (MSS) imagery acquired between 1972 and 1982. The 1990 dataset uses Landsat Thematic Mapper (TM) imagery acquired between 1984 and 1997. The 2000 dataset uses Landsat Enhanced Thematic Mapper Plus (ETM+) imagery acquired between 1999 and 2003. The 2005 dataset uses Landsat Thematic Mapper (TM) and Landsat Enhanced Thematic Mapper Plus (ETM+) imagery acquired between 2005 and 2008. The Landsat images are preselected to provide the best images (low cloud cover, greenest and closest to the preferred date) available to cover the Earth for 1975, 1990, 2000 and 2005. ASTER images can be selected from the entire ASTER archive to provide coverage from 2000 to the present.

Differences among images can have many causes. Satellite images provide practical documentation of the Earth through time, but to interpret images requires detective work to understand the underlying causes of the differences. The differences may represent different conditions of the land cover, such as seasonal changes or dryness. The differences may also represent changes in the land cover. Those changes may be fairly rapid, dramatic, and easily noticed: demolishing of a building, a fire sweeping through a forest or grassland, or the explosive eruption of a volcano. Some change is more gradual and even anticipated, such as the cyclical variations in the environment that influence numerous aspects of our lives: the alternating wet and dry seasons in tropical and subtropical regions, or the yearly progression of seasonal changes in the middle and higher latitudes. Still other landscape changes occur incrementally and, being less obvious, are detected only over a number of years or even decades or centuries: expanses of natural vegetation yielding to agricultural plots, encroaching urbanization transforming farmlands into housing tracts and shopping malls, or the shrinking of lakes as rivers are diverted for agricultural or urban use. Care must be taken to separate normal variability from actual change. The USGS Earthshots and UNEP Atlas of Our Changing Environment Web sites give many practical examples demonstrating the use of satellite images to document change.

The comparison of the same day for different years can reflect differences between wet and dry years or between early/late greenup/senescence. The dates of growing seasons vary significantly around the world. In arid regions, even an isolated rain storm can be seen as a local greening of the vegetation, one which may only remain green for a short period of time. Wetlands, particularly intermittent wetlands, can be among the most confusing to interpret. Along coastal areas, differences between low and high tides can give the impression of coastal erosion. River courses and discharges can change the appearances of the streambeds and sediment loads both in the rivers and offshore.

The pixel size of the image is an important influence on what can and cannot be seen. If you zoom in until you can see squares in the images, you are seeing the individual pixels. Landsat MSS has pixels that are 57 meters square, and Landsat TM and ETM+ have pixels that are 28.5 meters square. The ASTER pixels are 15 meters square. The ASTER images (60 x 60 km) cover about one-ninth the area of Landsat images (185 x 170 km). Cities and other detailed features look very different in the different images.

Dulles International Airport west of Washington, DC, opened in 1962. To the right of the North American locator map is an example of a full ASTER image that includes Washington, DC, on the Potomac River on the right edge with the airport as the bright spot to the left. The image on the right is a stack of the TerraLook images that include the airport. By clicking on the list of images under the index maps, you can change the right image. Notice the extra detail as the resolution of the images increases through time. Clicking on the image opens a larger view of the image.

Clicking through the satellite images from 1972 to 2005 shows the escalation of urbanization in areas of Maryland and Virginia surrounding Washington, DC. The dark green of wooded land and lighter greens of grass and agricultural plots increasingly give way to the white and purplish-blues of urban development in the Landsat images taken in fall and late spring. In the ASTER image captured in early spring, many of the fields remain bare, with the soil appearing in various shades of brown. Conspicuous growth has occurred in the areas around Dulles International Airport as well as along the major traffic arteries linking the airport to the nation's capital.

The considerable growth in these urban/suburban communities west of DC can be easily seen. Between DC and Dulles is an area that still has heavier vegetation cover, from Great Falls National Park on either side of the Potomac River in the north and continuing in a generally southerly direction. Large estates, one- to two-hectare (three- to five-acre) parcels, with enormous homes often referred to as "Mc Mansions," golf courses, commercial "parks" surrounded by substantial woodlands, and other regional parklands leave the area relatively less densely populated. Some further intensification of suburbanization will likely occur immediately along the major DC-Dulles highways, but it will be much slower pushing into the lower-density greenspace.

This series of satellite images from 1973 to 2007 tells the story of the loss of the Pararaenese forest, agricultural development, the construction of the world's largest hydroelectric dam, and the Iguazú Falls. Increasing agricultural activity, along with tourism-related development and associated growth of urbanization in the vicinity of Iguazú Falls, has encroached on the ecologically distinct subtropical forest shared by these countries. The progressive dwindling of dark green (forest) and expansion of light green and pinkish-purple (agricultural) and light bluish-grey (urban) colors clearly indicate the changing land cover as more than 90 percent of the forest has been converted to cropland—mostly soybeans and corn.

For hundreds of years, this region had little economic development, but in the 1960s rapid change began. Land reform in the 1960s started the development of eastern Paraguay and the conversion of forest to croplands. The opening of a free port in Brazil plus the construction of roads connecting Paraguay and Bolivia to Brazil across the Paraná opened the land to development. The newly constructed road can be seen crossing Paraguay to the Paraná River with the classic fishbone pattern of small fields along the side roads fanning out into the forest with the proportion of cropland to forest steadily increasing between 1973 and 2007. The opening of international markets and the tripling of the price of soybeans only accelerated this change.

In the 1989 image, the Itaipú dam and reservoir (constructed between 1973 and 1982) appears. Beneath this reservoir lies the Guairá Falls. Before being inundated by the reservoir, the Guairá had the greatest volume of any falls in the world. The Itaipú hydroelectric dam that flooded the falls and the surrounding area, is the world's largest, supplying over 25 percent of the electricity for Brazil and 75 percent for Paraguay.

In the 1973 image, the primary area of development is in Brazil. There, the only remaining forest land is within the Brazilian Iguaçu National Park. Along the Iguazú River, encroachment into the park can clearly be seen. However, following the construction of the dam, tourism opportunities related to the national parks became an increasingly important source of jobs, and as a result, the parks have become much better protected. Forest regrowth is visible in the later images.

In the September 2007 ASTER image most of the crops have not yet emerged. The higher resolution ASTER images provides the best views of the Iguazú Falls and the Itaipú Dam.

This series of satellite images from 1979 to 2006 tracks land use changes over nearly three decades of rapid population growth and accompanying urbanization in western Gambia, a West African country surrounded on three sides by Senegal. Banjul, the capital, lies at the easternmost tip of a peninsula along the mouth of the Gambia River and is separated from other cities in the Greater Banjul area by mangroves, which appear as the darkest green areas along the coasts. These mangroves remain largely intact as the growth of Banjul itself has been characterized by intensification rather than sprawl, and urbanization in the cities to the west of the mangroves has spread westward. However, a more-than-tripling of the population has led to replacement of woodlands (dark green) by croplands (lighter green) that are needed to feed the growing population of the expanding cities, which appear in shades of grey and white. While the mangroves currently remain intact, continued population growth with its increasing pressure on the land may pose a future threat to their survival.